PvP is like that everywhere, whether it's codex , HA or GvG..
Basically, they use multi launch and send in all their accounts, which helps them getting a lot of consecutive wins. They are leaving before halls to not get caught and aswell leave when a real team is forming..
The real problem there is that real players can't really do much while these can be syncing all day and reap rewards..
You form a party, beat one team then suddenly no opponent...( they went codex or GvG).. then you get bored and leave, they come again, etc...
That's why some players are asking for PvP formats playable or for some changes such as hench parties, which would at least help bored players get a match at anytime..